Re: [Qemu-devel] qemu-img convert cache mode for source

[Kevin Wolf <kwolf@redhat.com>]

Am 27.02.2014 um 17:12 hat Peter Lieven geschrieben:
> Am 27.02.2014 12:07, schrieb Kevin Wolf:
> > Am 27.02.2014 um 02:10 hat Fam Zheng geschrieben:
> >> On Wed, 02/26 16:41, Stefan Hajnoczi wrote:
> >>> On Wed, Feb 26, 2014 at 11:14:04AM +0100, Peter Lieven wrote:
> >>>> I was wondering if it would be a good idea to set the O_DIRECT mode for the source
> >>>> files of a qemu-img convert process if the source is a host_device?
> >>>>
> >>>> Currently the backup of a host device is polluting the page cache.
> >> Peter, can you give some more detailed explanation of the issue? An example or
> >> benchmark will help a lot. As I understand, one time scanning of a file doesn't
> >> promote the page cache to active list, so it probably won't hurt real hot cache
> >> at all, and will get replaced very soon.
> >>
> >> Considering readahead and page cache on metadata, I'm not sure if forcing
> >> O_DIRECT is a good idea.
> >>
> >>>    The problem is what to do for image formats.  An image file can be
> >>>    very fragmented so the readahead might not be a win.  Does this mean
> >>>    that for image formats we should tell the kernel access will be
> >>>    random?
> >>>
> >>>    Furthermore, maybe it's best to do readahead inside QEMU so that even
> >>>    network protocols (nbd, iscsi, etc) can get good performance.  They
> >>>    act like O_DIRECT is always on.
> >> Also, experience with booting a network backed guest can be greatly improved,
> >> because sometimes BIOS and bootloader are simple minded and load a kernel or
> >> initrd by sending thousands of 1 sector requests with iodepth=1, which means
> >> latency of network based IO hurts a lot.
> > I think I mentioned it a while ago, but our IDE emulation plays a role
> > in this as well. PIO requests are always handled sector by sector, no
> > matter how big the request was that we got from the BIOS.
> Yes, you have pointed that out before. How complicated is it to fix this?

I don't think it should be very complicated, but it basically means a
rewrite of the PIO read/write code, so there are chances to introduce
new bugs.

Kevin